Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

Abstract

Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (J d ) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to J d in OPDs. J d reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental J d values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

Ultrafast and broadband photodetectors based on a perovskite/organic bulk heterojunction for large-dynamic-range imaging

“…Serving as technical functional components for the translation of optical signals into electrical signals, photodetectors have received extensive attention and have been applied in various fields, including industrial production, military affairs, biochemical detection, optical communication, and scientific research [1][2][3][4][5][6][7][8][9][10] . The versatility and availability of photodetectors always depend on a few predominant factors: the photoresponse speed, sensitivity to lower brightness, detection band in which photodetectors can efficaciously detect light and dynamic range response [11][12][13][14][15][16] .…”

Color Determination from a Single Broadband Organic Photodiode

“…At the 900 nm optical mode close to the optical band edge R ≈ 0.08 A W −1 and D * ≈ 6.0 × 10 12 Jones (Figure S4 in the Supporting Information shows full spectra for R and D *). Relative to other reported OPDs,12,15,28,56,68 this device has a lower than average EQE (and thus R ) but a shot noise limited D * commensurate with the state‐of‐the‐art due to the low J d .…”

“…Figure 1c shows that the device (when scanned at 50 mV s −1 ) exhibits some hysteresis in dark conditions, which we predominantly attribute to displacement current (charge accumulation within intrinsic traps the in metal oxide EEL may also contribute). The device has J d ≈ 6 × 10 −7 mA cm −2 at −2 V applied bias, around five times lower than that recorded by Zhou et al for a similar PDPP3T:PC 70 BM device56 and amongst the lowest OPD J d values in recent literature comparisons 12,15. This is achieved by minimizing extrinsic leakage pathways such as pinholes65 by using an SU‐8 edge cover layer and having a comparatively thick (≈260 nm) active layer 25.…”

“…OPDs are currently attracting extensive interest2,3,12–16 due primarily to their compatibility with large‐area, low‐cost production17 onto lightweight flexible substrates, which facilitates applications such as wearable sensors,18,19 smart packaging,20,21 and certain medical technologies 18,22–24. Such devices usually operate under an applied reverse bias,15 and have wavelength sensitivity that can extend from the UV to the near infra‐red (NIR) 25,26. Various approaches have been employed to induce spectral sensitivity in OPDs without external color filtering 27.…”

“…IGZO is widely used for metal oxide thin film transistors (TFTs),37,43–47 and here forms an EEL since its −4.2 eV conduction band48,49 provides a downward energetic cascade (see energy level diagram50–53 in Figure 1b) for electrons under reverse bias (when the Au electrode is negative relative to the Ag electrode, see charge carrier directions in Figure 1b). 15 Furthermore, its deep (−7.5 eV) valence band43,48 suppresses unwanted hole injection, thus reducing dark current density J d 48,49. Crucially for this application, and in common with ZnO,35,39,40 the IGZO trap state density is influenced by the incident light color38,42 so can induce wavelength dependent electron extraction.…”